[SDK] Add darwin-x64 (Intel macOS) as the sixth per-platform package (#571)

[MGudgin]

Note

Stacked on #512. This PR's base is user/gudge/mxc-512-phase6-tests-docs,
so the diff shows only the single darwin-x64 commit. It must merge after
#512. Once #512 lands on main, retarget this PR's base to main (GitHub
auto-retargets when the base branch is deleted on merge).

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• This pull request is related to an issue.
• If this PR changes build commands, project architecture, or key conventions, I have updated .github/copilot-instructions.md.

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Summary

Adds Intel macOS (darwin-x64) support to the SDK's per-platform binary
delivery model introduced in #512, shipping mxc-exec-mac for
x86_64-apple-darwin as a sixth @microsoft/mxc-sdk-darwin-x64
optional-dependency package.

The Seatbelt backend is architecture-agnostic — it spawns
/usr/bin/sandbox-exec, which ships on every macOS — so there is no
arch-specific code in the backend (grep finds only cfg(target_os = "macos"),
no target_arch). #512 excluded darwin-x64 purely for delivery/CI reasons, not
capability; this PR reverses that as a build-and-ship change.

Closes #571

Details

• New platform package sdk/platform-packages/darwin-x64/ (manifest + README),
  mirroring darwin-arm64. darwin-x64 added to SUPPORTED_TUPLES in both
  platform.ts and the postinstall .cjs copy; meta optionalDependencies
  stamped to six packages via scripts/sync-platform-package-versions.js
  (lockfile refreshed).
• Unsupported-host messaging (platform.ts, helper.ts, postinstall-check.cjs)
  now describes the shipped set as win32/linux/darwin × (x64, arm64) — Intel
  macOS is no longer called unsupported.
• build-mac.sh already builds x86_64-apple-darwin; it now stages the binary
  into the darwin-x64 package.
• CI builds + tests the x64 slice natively on Intel runners. GHA
  Build.MacOS.Job.yml is a {arm64: macos-14, x64: macos-13} matrix; the GHA
  package + integration jobs gain darwin-x64 (integration on macos-13). ADO
  Mac.Build.Job.yml is parametrized (triplet/jobName/vmImage) and instantiated
  twice from 1ES.Build.Stages.yml (macOS-15-arm64 + macOS-13), with
  darwin-x64 added to the ADO package targets and integration matrix.
• Tests that asserted darwin-x64 was unsupported are flipped to "supported"; the
  release-completeness / version-sync "zombie package" fixtures now use a
  genuinely-fictional freebsd-x64.
• Docs updated to "six packages" (sdk/README.md, .azure-pipelines/README.md,
  .github/copilot-instructions.md).

Tests

• SDK unit: 284 tests, 276 pass / 8 skip / 0 fail (incl. the bidirectional
  SUPPORTED_TUPLES == on-disk dirs invariant, now 6/6). Script tests:
  32 pass. sync --check OK (6 packages). tsc --noEmit clean. All touched
  GHA + ADO YAML validates.
• Not verifiable locally (CI-only): the actual x86_64-apple-darwin
  build/test on the Intel runners, darwin-x64 packaging (release-completeness
  gate over real artifacts), the Intel integration E2E, and ADO ESRP
  signing/notarization of the x64 slice.

Reviewer notes

• ⚠️ macos-13 is Apple's last hosted Intel runner and is being deprecated
  (GitHub + Azure). The x64 build/test/integration lanes depend on it; when it
  is retired they will need revisiting (cross-compile + Rosetta-on-arm64, or
  removal). Calling this out as a known maintenance risk, not a blocker.
• Please confirm on a pipeline run that the x64 mac build, ESRP signing of
  mxc-exec-mac (x86_64), and the darwin-x64 packaging + integration lanes
  behave as expected — these are the parts that cannot be exercised off-pipeline.

Microsoft Reviewers: Open in CodeFlow